The present application is based on Japanese priority application No. 11-315424 filed on Nov. 5, 1999, the entire contents of which are hereby incorporated by reference.
The present invention generally relates to semiconductor devices and more particularly to an optical semiconductor device using a SiGeC mixed crystal for an active region thereof and an opto-electronic integrated circuit using such an optical semiconductor device.
In an optical semiconductor device, it is necessary to use a semiconductor material having a direct-transition type for the active region thereof. A direct-transition type semiconductor material causes a transition as a result of interaction with optical radiation, without intervention of phonons.
Thus, conventional optical semiconductor devices have used a III-V compound semiconductor material such as GaAs for the active region or active layer thereof. On the other hand, such a III-V semiconductor material has a drawback in that it is difficult to achieve a lattice matching with a Si substrate, which is used conventionally as the substrate of semiconductor integrated circuit. Thus, it has been difficult to construct an opto-electronic integrated circuit in which a III-V optical semiconductor device and a Si semiconductor device are integrated on a common Si substrate. Si is not a material of the direct-transition type band structure.
Meanwhile, it has been known from a theoretical calculation, in the system of SiGe mixed crystal, that the band structure of the SiGe mixed crystal changes to the direct-transition type when a Si atomic layer and a Ge atomic layer are stacked alternately in the (001) direction to form a hypothetical superlattice structure in the SiGe mixed crystal. In such a hypothetical SiGe ordered mixed crystal structure, the bottom edge of the conduction band appearing on the xcex94 axis coincides with the xcfx84 point as a result of the zone-folding effect of the energy band.
On the other hand, it has been known in the foregoing ordered SiGe mixed crystal system that the bottom conduction band edge on the xcex94 axis contains a p-orbital component with a large proportion exceeding 50% and that the number of the states of the carriers for the bottom edge of the conduction band is increased in the Si atomic layers while the number of the states for the top edge of the valence band is increased in the Ge atomic layers. In view of the situation noted above, it is difficult to realize a large probability of transition between the valence band edge and the conduction band edge in such a SiGe ordered mixed crystal.
Further, a conventional optical semiconductor device using a SiGe mixed crystal has suffered from the problem of poor carrier confinement caused as a result of small band discontinuity formed at the interface between a Si barrier layer and the SiGe active layer. Because of the poor carrier confinement, it has been difficult to achieve a laser oscillation in the optical semiconductor device using a SiGe mixed crystal for the active layer. Further, it is not possible in such a conventional semiconductor device to achieve a lattice matching between the SiGe mixed crystal and the Si substrate because of the large discrepancy of lattice constant between Si and Ge.
In view of the situation noted above, the inventor of the present invention has made a prediction that a superlattice structure of a SiGeC mixed crystal may have a band structure of direct-transition type as a result of the zone-folding effect of energy band in such a manner that the L point on the bottom edge of the conduction becomes coincident with the xcfx84 point, provided that the SiGeC mixed crystal has a superlattice structure in which Si atomic layers and Si1xe2x88x92xCx atomic layers are repeatedly stacked in the  less than 111 greater than  direction such that a unit of repetition contains therein the Si atomic layers and the Si1xe2x88x92xCx atomic layers in total number of a multiple of four (Ohfuti, M., et al., Inst. Phys. Conf. Ser. No.162: Chapter 7, paper presented at 25th Int. Sump. Compound Semiconductors, Nara, Japan, Oct. 12-16, 1998. pp.325-330). Further, the inventor confirmed the foregoing prediction by means of the energy calculation based on the first principle (Ohfuti, M., op. cit.).
On the other hand, the foregoing discovery by the inventor of the present invention is made for the superlattice structure of the SiGeC mixed crystal in which the Ge and C are xe2x80x9cordered.xe2x80x9d In view of the fact that the foregoing theoretical prediction of the inventor is based on the zone folding effect occurring in the ordered structure, it has not been apparent whether or not a SiGeC mixed crystal having a random arrangement of Si, Ge and C also has a direct-transition band structure similar to the one derived for the ordered SiGeC mixed crystal structure.
In the actual device fabrication, it is predicted that the formation of an ordered SiGeC mixed structure would need an atomic layer epitaxy (ALE) process in which atomic layers of Si, Ge and C are grown one layer by one layer on a properly chosen crystal surface under a properly chosen condition, while the ALE process for such an ordered SiGeC mixed crystal is complex and increases the fabrication cost of the optical semiconductor device.
Accordingly, it is a general object of the present invention to provide a novel and useful optical semiconductor device and an opto-electronic integrated circuit using such an optical semiconductor device wherein the foregoing problems are eliminated.
Another object of the present invention is to provide an efficient optical semiconductor device having an active region of a SiGeC mixed crystal in which Si atoms, Ge atoms, and C atoms are mixed at random.
Another object of the present invention is to provide an optical semiconductor device, comprising:
a Si substrate having a first conductivity type;
an active layer of a SiGeC mixed crystal formed over said Si substrate in an epitaxial relationship therewith, said SiGeC mixed crystal containing Si atoms, Ge atoms, and C atoms in a random atomic arrangement;
a cladding layer formed on said active layer with a second, opposite conductivity type;
a first electrode provided in electrical contact with said Si substrate; and
a second electrode provided in electrical contact with said cladding layer.
Another object of the present invention is to provide an opto-electronic integrated circuit, comprising:
a Si substrate having a first region having a first conductivity type and a second region;
an optical semiconductor device formed on said first region of said Si substrate,
said optical semiconductor device comprising: a lower cladding layer formed epitaxially on said first region of said Si substrate with said first conductivity type; an active layer of a SiGeC mixed crystal formed on said lower cladding layer epitaxially, said SiGeC mixed crystal containing Si atoms, Ge atoms, and C atoms in a random atomic arrangement; an upper cladding layer formed epitaxially on said active layer with a second, opposite conductivity type; a first electrode provided in electrical contact with said Si substrate; and a second electrode provided in electrical contact with said second epitaxial layer;
an optical waveguide formed on said second region of said Si substrate; and
an electronic device formed on a third region of said Si substrate.
According to the present invention, a direct-transition type group IV semiconductor material is realized in the form of a SiGeC mixed crystal having a random arrangement for the Si, Ge and C atoms. By introducing C into the SiGeC mixed crystal, it is possible to tune the bandgap energy to the value of 0.95 eV or less corresponding to the 1.3 xcexcm or longer wavelength band. For example, it is possible to tune the bandgap of the SiGeC mixed crystal to the 1.3 xcexcm band by setting the atomic fraction of the Ge atoms to about 0.24 and the atomic fraction of the C atoms to about 0.03. By setting the atomic fraction of the Ge atoms to about 0.32 and the atomic fraction of the C atoms to about 0.04, the bandgap of the SiGeC mixed crystal is tuned to the 1.8 xcexcm band. Further, by setting the atomic fraction of the Ge atoms to about 0.29 and the atomic fraction of the C atoms to about 0.036, the bandgap of the SiGeC mixed crystal is tuned to the 1.55 xcexcm band. Thus, the SiGeC mixed crystal can be used to construct an optical semiconductor device such as a laser diode for optical telecommunication.
In view of the small bandgap of the SiGeC mixed crystal of the present invention, an effective carrier confinement is achieved when the SiGeC mixed crystal is used for the active layer of an optical semiconductor device. In the case the optical semiconductor device is used as a laser diode, a stable laser oscillation becomes possible at a room temperature environment, without providing a cooling device.
In view of the fact that the Si atoms, C atoms and Ge atoms are mixed at random in the SiGeC mixed crystal of the present invention, the optical semiconductor device can be fabricated easily by using a CVD process.
In the present invention, the SiGeC mixed crystal can be formed to achieve a lattice matching to the Si substrate by controlling the C concentration as necessary.
As the optical semiconductor device is constructed on a Si substrate, it is easy to construct an opto-electronic integrated circuit by providing a semiconductor device and an optical waveguide commonly on the Si substrate, the random SiGeC mixed crystal can also be used in such an optical waveguide as the core layer or optical waveguide layer. In such a case, it is preferable to modify the composition of the SiGeC mixed crystal forming the optical waveguide layer with respect to the composition of the SiGeC mixed crystal forming the active layer of the laser diode so as to avoid optical absorption of the laser beam produced by the laser diode.
Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings.